Trauma is the leading cause of death of young people in the U.S. (150,000/year). Most trauma deaths result either from insufficient tissue perfusion, due to excessive blood loss, or the development of inflammation, infection and end organ damage following resuscitation. Current treatment plans for hypovolemic shock rely on massive and rapid infusion of crystalloid fluids to raise cardiac output. It is now recognized that excessive volume resuscitation may increase blood loss and the reperfusion injury that contributes to the morbidity of hypovolemic shock. As such, the development of alternative strategies for the treatment of traumatic blood loss will be critical for the improvement of patient outcomes following hypovolemic shock; progressive hemorrhage produces a biphasic response. Increases in heart rate and sympathetic activity maintain blood pressure in the initial compensatory phase. These compensatory responses suddenly abate after significant blood loss (20-30%), resulting in hypotension, bradycardia and sympathoinhibition. We recently discovered that drugs that activate serotonin 5-HTIA receptors rapidly reverse the hypotensive and sympathoinhibitory responses to hemorrhage in conscious rats. Both central and systemic administration of 5-HTIA receptor agonists effectively raises blood pressure after either acute or sustained blood loss. These results indicate that 5-HTIA agonists could provide a promising therapy for hypovolemic shock. However, it is not known how 5-HTIA receptor activation increases arterial pressure or if the hemodynamic responses to agonist administration provide a beneficial effect on tissue perfusion. More importantly, it is not known if activation of 5-HTIA receptors can delay the transition from hypovolemic shock to circulatory collapse. Studies proposed in this project will determine the autonomic and hemodynamic effects of selective 5-HTIA agonists following sustained hypotensive hemorrhage to assess their impact on perfusion. Additional studies will assess the central nervous system mechanisms responsible for pressor effects of selective 5-HTIA agonists during hemorrhage. We will also assess the ability of clinically available 5-HT1A agonists to delay the transition from hypovolemic shock to circulatory collapse.